1. Field of the Invention
The present invention relates generally to the field of radiation imaging, and in particular to radiation imaging devices having an array of image cells.
2. Related Art
A variety of imaging devices comprising an array of image cells are currently known. A charged coupled image sensor (also known as a charged coupled device (CCD)) is one example of such an imaging device. A CCD type device operates in the following way. Charge is accumulated within a depletion region created by an applied voltage. For each pixel (image cell) the depletion region has a potential well shape and constrains electrons under an electrode gate to remain within the semiconductor substrate. Voltage is then applied as a pulse to the electrode gates of the CCD device to clock each charge package to an adjacent pixel cell. The charge remains inside the semiconductor substrate and is clocked through, pixel by pixel, to a common output. During this process, additional charge cannot be accumulated.
Another type of imaging device which is known is a semiconductor pixel detector which comprises a semiconductor substrate with electrodes which apply depletion voltage to each pixel position and define a charge collection volume. Typically, simple buffer circuits read out the electric signals when a photon is photo-absorbed or when ionizing radiation crosses the depletion zone of the substrate. Accordingly pixel detectors of this type typically operate in a pulse mode, the numbers of hits being accumulated externally to the imaging device. The buffer circuits can either be on the same substrate () as the charge collection volumes, as disclosed in European Patent Application EP-A-0287197, or on a separate substrate that is mechanically bonded to a substrate having the charge collection volumes in accordance with, for example, the well known bump-bonding technique, as disclosed in European Patent Application EP-A-0571135.
Another type of imaging device is described in International patent application WO95/33332, which describes an Active-pixel Semiconductor Imaging Device (ASID). The ASID comprises an array of pixel cells including a semiconductor substrate having an array of pixel detectors and a further array of pixel circuits. The pixel detectors generate charge in response to incident radiation. Each pixel circuit is associated with a respective pixel detector and accumulates charge resulting from radiation incident on the pixel detector. The pixel circuits are individually addressable and comprise circuitry which enables charge to be accumulated from a plurality of successive radiation hits on the respective pixel detectors. The device operates, for example, by accumulating charge on a gate of a transistor. Accordingly, analog storage of the charge value is obtained. At a determined time, the charge from the pixel circuits can be read out and used to generate an image based on the analog charge values stored in each of the pixel circuits.
CCD devices suffer from several disadvantages, including limited dynamic range due to the limited capacity of the potential well inside the semiconductor substrate, and inactive times during which an image is read out. Pulse counting semiconductive pixel devices also suffer from limited dynamic range. As these devices read the pixel contact when a hit is detected, they suffer from saturation problems at high counting rates. The semiconductor pixel device according to WO95/33332 provides significant advantages over the earlier prior art by providing a large dynamic range for the accumulation of images.
However, CCD imaging devices and imaging devices of the type described in WO95/33332 suffer from a potential disadvantage in that the output signals from the individual pixel cells represent the accumulation of radiation intensity at that pixel cell between readout times. This means that radiation hits of varying energies could lead to an inaccurate count of the number of radiation hits. For example, a relatively small number of higher energy radiation hits would give the same output signal as a higher number of lower energy radiation hits (for example, scattered radiation hits).
Embodiments of the present invention seek to mitigate the problems of known imaging devices described above.
According to an embodiment of the present invention, an imaging device for imaging radiation comprises an imaging cell array including an array of detector cells which generate charge in response to incident radiation and an array of image cell circuits. Each image cell circuit is associated with a respective detector cell, and the image cell circuit comprises counting circuitry for counting plural radiation hits incident on the associated detector cell.
In accordance with this embodiment, it is possible to provide an accurate count of the number of hits on each image cell even at high intensities (e.g., high hit rates) by counting each incident radiation hit on each image cell. An example of a device implementing such an embodiment avoids readout bandwidth problems by counting at the image cells and allowing readout at a much lower rate than would be the case with typical pulse counting devices where the signals need to be read out of the device before being counted.
Embodiments of the present invention also simplify the processing necessary on reading out the contents of the array of image cells. Typically the image cells will be pixels of a two-dimensional array; however, the image cells could also be strips in a strip cell in an imaging strip device.
In accordance with another embodiment, an image cell circuit may comprise threshold circuitry connected to receive signals generated in an associated detector cell, with threshold values dependent on incident radiation energy. Counting circuitry may be connected to the threshold circuitry for counting only radiation hits within a predetermined energy range or ranges.
By providing thresholding of the signals at each image cell, it is possible to reduce the storage capacity of the counter which is required, and also accurately to record the number of radiation hits of a desired energy. By recording hits of only selected radiation energies, it is possible, for example, to ensure that only directly incident rays are counted, and counting of hits resulting from scattered, reflected or defracted rays (which will have a lower energy) can be avoided. By use of this technique, the overall quality and resolution of an image can be greatly improved.
In accordance with another embodiment, the threshold circuitry comprises first and second comparators for comparing an input signal value to upper and lower threshold values, respectively. By using two comparators, it is possible to identify signals within a range having upper and lower bounds. With a single threshold comparator, it would be possible to obtain storage of signals either above, or below, that threshold. A trigger circuit responsive to outputs of the first and second comparators may be provided to increment a count in a counter in response to input signals having a value between the first and second threshold values. This may be achieved, for example, by providing the trigger circuitry with a flip-flop having a clock input connected via delay circuit to an output of the second comparator, a data signal input connected via a one shot circuit to an output of the first comparator and an output connected to the counter. To enable the image accumulation process to be substantially continuous, the output of the counter is connectable to a loadable shift register. The shift register of an image cell circuit is chained (in series) with respective shift registers of further image cell circuits of the array.
According to an alternate embodiment, a first counter is responsive to the output of the first comparator, and a second counter is responsive to an output of the second comparator. In order to enable the image accumulation process to be substantially continuous, in this embodiment the output of the first counter is connected to a first loadable shift register and the output of the second counter is connected to a second loadable shift register. The first and second loadable shift registers of an image cell circuit are chained together (in series or possibly at least partially in parallel) and with shift registers of further image cell circuits of the array. The shift registers of the image cell circuits may be connected to an output shift register arrangement external to the array. Further, the output shift register may have a dual x-y configuration.
According to another embodiment of the invention, an imaging system may be configured using one or more imaging devices in accordance with the embodiments described above.